Many thin film processes use plasma processes to facilitate the rapid and accurate fabrication of minute structures with desired properties. For instance, in the manufacture of integrated circuits, materials such as silicon dioxide, silicon nitride, polysilicon, metal, metal silicide, and monocrystalline silicon are etched in predefined patterns to form gates, vias, contact holes, trenches, and/or interconnect lines. In the etching process, a patterned mask composed of silicon oxide or silicon nitride (hard mask) or photoresist polymer, is formed on the substrate by conventional photolithographic methods. The exposed portions of the underlying material that lie between the features of the patterned mask are etched by capacitive or inductively coupled plasmas of etchant gas.
The plasma process usually involves placing the substrate in a vacuum chamber, introducing process gases and applying radio-frequency (RF) power, typically 0.1 to 200 MHz, to create the plasma. The plasma consists of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed. The plasma reaction has many inputs, including RF power, gas type and flow rates, chamber pressure, substrate and wall temperatures, chamber wall conditions, electrode spacing, and so on. The chamber configuration and chemistry used is chosen according to the desired process. For example, plasmas are used to etch dielectrics in semiconductor manufacture using specific plasma chamber designs such as Reactive Ion Etching (RIE) or Inductively Coupled Plasma (ICP) and using etching gases such as CHF3, CF4, O2 and so on.
Capacitance coupled plasma chambers or reactors are usually constructed with a pair of parallel plate electrodes facing each other, spaced apart in parallel, and placed inside a vacuum chamber. As the term “capacitance coupled plasma” implies, the electrodes form a capacitor, typically of the parallel plate type. The most fundamental type is simply two flat plates of opposite electrical polarity and is often referred to as a “planar diode.” The electrodes may be arranged in a variety of geometric configurations, including configurations having curved surfaces, such as concentric parallel cylinders or concentric spheres with parallel tangents. An external electric field, either DC or AC, is applied to the opposite electrodes. Under low pressure and with proper spacing between the electrodes, a stable plasma can be generated and maintained by first ionizing and then creating a glow discharge in gas flowing between the electrodes. Multiple pairs of alternating polarity parallel plates can be spaced apart and/or stacked together to form multiple regions where plasma discharge may occur. Such capacitance coupled plasma reactors have been widely used in a variety of industries for applications such as substrate etching, substrate cleaning, substrate film deposition, gas treatment, ion beam source and for various chemical reactions.
During the etching processes, etchant residue (e.g., polymer material) often deposits on the walls and other component surfaces inside the etching chamber. The composition of the etchant residue depends upon the chemical composition of the vaporized species of etchant gas, the material being etched, and the mask layer on the substrate. For example, when tungsten silicide, polysilicon or other silicon-containing layers are etched, silicon-containing gaseous species are vaporized or sputtered from the substrate; similarly, etching of metal layers results in vaporization of metal species. In addition, the mask layer on the substrate is also partially vaporized by the etchant gas to form gaseous hydrocarbon or oxygen species. The vaporized and gaseous species condense to form etchant residue comprising polymeric byproducts composed of hydrocarbon species from the resist; gaseous elements such as fluorine, chlorine, oxygen, or nitrogen; and elemental silicon or metal species depending on the composition of the substrate being etched. The polymeric byproducts deposit as thin layers of etchant residue on the walls and components in the chamber. The composition of the etchant residue typically varies considerably across the chamber surface depending upon the composition of the localized gaseous environment, the location of gas inlet and exhaust ports, and the geometry of the chamber.
Two important parameters in plasma-assisted processes are the plasma density and the ion energy of the species that impact on the wafer surface. Control of these parameters is important for controlling the quality of the structures being formed. However, it is difficult to control these parameters because they are generally not directly measured in conventional plasma chambers. Rather, the power that is supplied to the plasma is usually the parameter that is controlled. The supplied power, however, cannot be used as an indicator of the plasma density and ion energy because the impedance of the load to which the power is delivered is not constant with time. The impedance of the load changes because, as previously mentioned, during the manufacturing process, by-products of the reaction process (e.g., polymers) are deposited on the walls of the chamber. The by-products change the impedance, particularly the resistance, of the load. Of course, the effective power that is delivered to the plasma changes as the impedance of the load changes. Since the effective power changes, the plasma density and ion energy also change.